An electrophotographic light-sensitive material may have various structures depending upon the characteristics required or an electrophotographic process to be employed.
Typical electrophotographic light-sensitive materials widely employed comprise a support having provided thereon one photoconductive layer and an insulating layer on the surface thereof.
The electrophotographic light-sensitive material comprising a support and at least one photoconductive layer formed thereon is used for the image formation by an ordinary electrophotographic process including electrostatic charging, imagewise exposure, development, and, if desired, transfer.
Furthermore, a process using an electrophotographic light-sensitive material as an offset master plate precursor for direct plate making is widely practiced. In particular, a direct electrophotographic lithographic plate has recently become important as a system for printing in the order of from several hundreds to several thousands prints having a high image quality.
Under these circumstances, binder resins which are used for forming the photoconductive layer of an electrophotographic light-sensitive material are required to be excellent in the film-forming properties by themselves and the capability of dispersing photoconductive powder therein. Also, the photoconductive layer formed using the binder resin is required to have satisfactory adhesion to a base material or support. Further, the photoconductive layer formed by using the binder resin is required to have various excellent electrostatic characteristics such as high charging capacity, small dark decay, large light decay, and less fatigue due to prior light-exposure and also have an excellent image forming properties, and the photoconductive layer stably maintains these electrostatic properties in spite of the fluctuation in humidity at the time of image formation.
Further, extensive studies have been made for lithographic printing plate precursors using an electrophotographic light-sensitive material, and for such a purpose, binder resins for a photoconductive layer which satisfy both the electrostatic characteristics as an electrophotographic light-sensitive material and printing properties as a printing plate precursor are required.
It has been found that the chemical structure of binder resin used in a photoconductive layer which contains at least an inorganic photoconductive substance, a spectral sensitizing dye and a binder resin has a great influence upon the electrostatic characteristics as well as smoothness of the photoconductive layer. Among the electrostatic characteristics, dark charge retention rate (D.R.R.) and photosensitivity are particularly affected.
Techniques for improvements in smoothness and electrostatic characteristics of a photoconductive layer by using a resin having a low molecular weight and containing a component having an acidic group are described, for example, in JP-A-63-217354 and JP-A-3-181948, (the term "JP-A" as used herein means an "unexamined published Japanese Patent Application"), U.S. Pat. Nos. 4,968,572, 5,021,311, 5,134,051, 4,495,407, 5,063,130, 5,183,721, 5,147,752, 5,089,368 and 5,154,997, and EP-A-0432727. It is presumed that even when the stoichiometric defect of the inorganic photoconductive substance varies to some extents, a relatively stable interaction between the inorganic photoconductive substance, spectral sensitizing dye and resin may be maintained since these various low molecular weight resins have the sufficient adsorptive domain by their functions and mechanisms. Of these resins, the graft type copolymer and AB block copolymer can provide a relatively stable performance even when ambient conditions are fluctuated.
Further, techniques for improving a mechanical strength of a photoconductive layer by using the above described low molecular weight resin containing an acidic group together with a medium to high molecular weight resin are described, for example, in U.S. Pat. Nos. 5,183,720, 5,104,759, 5,124,221, 5,135,830, 5,116,710, 5,206,104, 5,183,721, 5,202,208, 5,206,105, 5,135,831 and 5,089,368, EP-A-0458298, JP-A-3-188454, JP-A-3-223761, JP-A-3-217846, JP-A-4-14050, JP-A-4-15654, JP-A-4-20969 and JP-A-4-25851.
However, it has been found that, even in a case of using these resins or a combination of these resins, a problem may arise sometimes on a mechanical strength of the photoconductive layer formed in practice. For instance, when the light-sensitive material is supplied to a duplicating machine or a plate-making machine in the form of a sheet or roll, a surface of the photoconductive layer is always in contact with a rear surface of the light-sensitive material and it is unavoidable that the surface of the photoconductive layer is intensely rubbed sometimes. As the result, a problem arises in a duplicated image formed through an electrophotographic process in that cut of image or background stain in the non-image area occurs in the portion where the surface of the photoconductive layer has been rubbed.
Also, it is desirable to maintain good and stable reproducibility of duplicated image in the case of greatly fluctuating the ambient conditions from high-temperature and high-humidity to low-temperature and low-humidity. In particular, in a scanning exposure system using a semiconductor laser beam, the exposure time becomes longer and also there is a restriction on the exposure intensity as compared with a conventional overall simultaneous exposure system using a visible light, and hence a higher performance has been required for the electrostatic characteristics, in particular, the dark charge retention characteristics and photosensitivity.
Moreover, it has been desired to develop a technique which can faithfully reproduce highly accurate images of continuous gradation as well as images composed of lines and dots using a liquid developer.
However, as a result of evaluation on practical image reproducibility of hitherto known light-sensitive materials it is found that some problems arise in that gradation of the duplicated image becomes soft, in that edge marks of originals pasted up appear and in that unevenness of density occurs at a continuous gradation portion of the duplicated image practically formed in spite of good electrostatic characteristics.
Particularly, when the scanning exposure system using a semiconductor laser beam is applied to hitherto known light-sensitive materials for electrophotographic lithographic printing plate precursors, unevenness of density at a continuous gradation portion occurs on prints or unevenness of white spots at a high density image portion on prints occurs under low humidity conditions. Specifically, in the hitherto known technique, the improved image reproducibility which is achieved by means of the low molecular weight resin may be sometimes deteriorated by the medium to high molecular weight resin used together therewith. In fact, it has been found that an electrophotographic light-sensitive material having a photoconductive layer wherein the above described known resins are used in combination may cause a problem on reproducibility of the above described highly accurate image (particularly, an image of continuous gradation) or on image forming performance in case of using a scanning exposure system with a laser beam of low power.
The present invention has been made for solving the problems of hitherto known electrophotographic light-sensitive materials as described above.
An object of the present invention is to provide an electrophotographic light-sensitive material having an extremely good anti-abrasion property and excellent reproducibility of duplicated images and giving clear good images.
Another object of the present invention is to provide an electrophotographic light-sensitive material having stable and excellent electrostatic characteristics and giving clear good images even when the ambient conditions during the formation of duplicated images are fluctuated to low-temperature and low-humidity or to high-temperature and high-humidity.
A further object of the present invention is to provide a CPC electrophotographic light-sensitive material having excellent electrostatic characteristics and showing less environmental dependency.
A further object of the present invention is to provide an electrophotographic light-sensitive material effective for a scanning exposure system using a semiconductor laser beam.
A still further object of the present invention is to provide an electrophotographic lithographic printing plate precursor having excellent electrostatic characteristics (in particular, dark charge retention characteristics and photosensitivity), capable of reproducing a faithfully duplicated image to the original (in particular, a highly accurate image of continuous gradation), forming neither overall background stains nor dotted background stains of prints, and showing excellent printing durability.
Other objects of the present invention will become apparent from the following description.